U.S. patent application number 12/825117 was filed with the patent office on 2010-12-30 for self-aligned spacer multiple patterning methods.
This patent application is currently assigned to Rohm and Haas Electronics Materials LLC. Invention is credited to Young Cheol Bae, Thomas Cardolaccia, Yi Liu.
Application Number | 20100330498 12/825117 |
Document ID | / |
Family ID | 42697262 |
Filed Date | 2010-12-30 |
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United States Patent
Application |
20100330498 |
Kind Code |
A1 |
Bae; Young Cheol ; et
al. |
December 30, 2010 |
SELF-ALIGNED SPACER MULTIPLE PATTERNING METHODS
Abstract
Self-aligned spacer multiple patterning method are provided. The
methods involve alkaline treatment of photoresist patterns and
allow for the formation of high density resist patterns. The
methods find particular applicability in semiconductor device
manufacture.
Inventors: |
Bae; Young Cheol; (Weston,
MA) ; Cardolaccia; Thomas; (Needham, MA) ;
Liu; Yi; (Wayland, MA) |
Correspondence
Address: |
Jonathan D. Baskin;Rohm and Haas Electronic Materials LLC
Patent Department, 455 Forest Street
Marlborough
MA
01752
US
|
Assignee: |
Rohm and Haas Electronics Materials
LLC
Marlborough
MA
|
Family ID: |
42697262 |
Appl. No.: |
12/825117 |
Filed: |
June 28, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61269600 |
Jun 26, 2009 |
|
|
|
61281553 |
Nov 19, 2009 |
|
|
|
61281681 |
Nov 19, 2009 |
|
|
|
Current U.S.
Class: |
430/270.1 ;
430/312 |
Current CPC
Class: |
G03F 7/0035 20130101;
H01L 21/0337 20130101; H01L 21/0273 20130101; H01L 21/0338
20130101; G03F 7/40 20130101 |
Class at
Publication: |
430/270.1 ;
430/312 |
International
Class: |
G03F 7/004 20060101
G03F007/004; G03F 7/20 20060101 G03F007/20 |
Claims
1. A self-aligned spacer multiple patterning method, comprising:
(a) providing a semiconductor substrate comprising one or more
layers to be patterned; (b) applying a first layer of a first
photosensitive composition over the one or more layers to be
patterned, wherein the first photosensitive composition comprises a
first resin component and a first photoactive component; (c)
exposing the first layer to activating radiation through a
patterned photomask; (d) developing the exposed first layer to form
a resist pattern; (e) heat treating the resist pattern in a
hardbake process; (f) treating the hardbaked resist pattern with a
material effective to make alkaline a surface of the resist
pattern; (g) applying a second layer of a second photosensitive
composition over the one or more layers to be patterned and in
contact with the alkaline surface of the resist pattern, the second
photosensitive composition comprising a second resin component and
a photoacid generator; (h) exposing the second layer to activating
radiation; and (i) developing the exposed second layer to form
spacers over the one or more layers to be patterned, the spacers
comprising portions of the second layer not removed during the
second layer development.
2. The method of claim 1, wherein the step of exposing the second
layer is a flood exposure.
3. The method of claim 1, wherein the steps of exposing and
developing the second layer exposes and removes, respectively, the
resist pattern.
4. The method of claim 1, further comprising removing the resist
pattern after the steps of exposing and developing the second
layer.
5. The method of claim 1, further comprising patterning one or more
layer underlying the spacers using the spacers as a mask.
6. The method of claim 1, further comprising: (i) forming a spacer
layer over sidewalls of the spacers, wherein the spacer layer is of
a different material than the second layer; and (j) selectively
removing the first spacers from the substrate, leaving second
spacers formed from the spacer layer over the one or more layers to
be patterned.
7. The method of claim 6, wherein the heat treating the resist
pattern is conducted at a temperature of about 150.degree. C. or
higher.
8. The method of claim 1, wherein treating the resist pattern with
the material effective to make alkaline a surface of the resist
pattern comprises treating the resist pattern with an alkaline
material and a surfactant.
9. The method of claim 1, wherein treating the resist pattern with
the material effective to make alkaline a surface of the resist
pattern comprises treating the resist pattern with a primary or
secondary amine.
10. A coated substrate, comprising: (a) a semiconductor substrate
comprising one or more layers to be patterned; (b) a resist pattern
over the one or more layers to be patterned, the resist pattern
having an alkaline surface; and (c) a photosensitive layer on
sidewalls of the first resist pattern, the photosensitive layer
comprising a composition comprising a resin component and a
photoacid generator component.
Description
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119(e) to U.S. Provisional Application Nos.
61/269,600, filed Jun. 26, 2009 and 61/281,553, filed Nov. 19, 2009
and 61/281,681 filed Nov. 19, 2009, the entire contents of which
applications are incorporated herein by reference.
[0002] This invention relates generally to the manufacture of
electronic devices. More specifically, this invention relates to
methods of forming electronic devices using self-aligned spacer
lithographic techniques. The invention finds particular use in the
manufacture of semiconductor devices for forming high-density
lithographic patterns and features. Double or higher order
patterning can be performed in accordance with the invention.
[0003] In the semiconductor manufacturing industry, photoresist
materials are used for transferring an image to one or more
underlying layers, such as metal, semiconductor or dielectric
layers, disposed on a semiconductor substrate, as well as to the
substrate itself. To increase the integration density of
semiconductor devices and allow for the formation of structures
having dimensions in the nanometer range, photoresists and
photolithography processing tools having high-resolution
capabilities have been and continue to be developed.
[0004] One approach to achieving nm-scale feature sizes in
semiconductor devices is the use of short wavelengths of light, for
example, 193 nm or less, during exposure of chemically amplified
photoresists. Immersion lithography effectively increases the
numerical aperture of the lens of the imaging device, for example,
a scanner having a KrF or ArF light source. This is accomplished by
use of a relatively high refractive index fluid (i.e., an immersion
fluid) between the last surface of the imaging device and the upper
surface of the semiconductor wafer. The immersion fluid allows a
greater amount of light to be focused into the resist layer than
would occur with an air or inert gas medium.
[0005] The theoretical resolution limit as defined by the Rayleigh
equation is shown below:
R = k 1 .lamda. NA ##EQU00001##
where k.sub.1 is the process factor, .lamda. is the wavelength of
the imaging tool and NA is the numerical aperture of the imaging
lens. When using water as the immersion fluid, the maximum
numerical aperture can be increased, for example, from 1.2 to 1.35.
For a k.sub.1 of 0.25 in the case of printing line and space
patterns, 193 nm immersion scanners would only be capable of
resolving 36 nm half-pitch line and space patterns. The resolution
for printing contact holes or arbitrary 2D patterns is further
limited due to the low aerial image contrast with a dark field mask
wherein the theoretical limit for k.sub.1 is 0.35. The smallest
half-pitch of contact holes is thus limited to about 50 nm. The
standard immersion lithography process is generally not suitable
for manufacture of devices requiring greater resolution.
[0006] In an effort to achieve greater resolution and to extend
capabilities of existing manufacturing tools, various double
patterning techniques have been proposed. One such technique is
self-aligned double patterning (SADP). In this process, a spacer
layer is formed over pre-patterned lines. This is followed by
etching to remove all spacer layer material on horizontal surfaces
of the lines and spaces, leaving behind only material on the
sidewalls of the lines. The original patterned lines are then
etched away, leaving behind the sidewall spacers which are used as
a mask for etching one or more underlying layers. Since there are
two spacers for every line, the line density is effectively
doubled. The spacer approach is unique as compared with the
previously described techniques in that only one lithographic
exposure is required. As a result, issues associated with overlay
between successive exposures can be avoided.
[0007] U.S. Patent Application Pub. No. 2009/0146322A1 discloses a
self-aligned spacer double patterning method in which the
sacrificial first pattern can comprise polysilicon, while the
spacer layer can comprise silicon nitride, silicon oxide or silicon
oxynitride. That document further discloses that such materials are
coated by deposition processes, such as physical vapor deposition,
chemical vapor deposition, electrochemical deposition, molecular
beam epitaxy or atomic layer deposition. These deposition
processes, however, require repeated etch steps which may not be
cost effective. For example, when silicon oxynitride is deposited
as a sidewall spacer material, the process can involve numerous
steps such as resist trim etch, pattern transfer etch to amorphous
carbon layer, cleaning, deposition of nitride over the amorphous
carbon template, etch to open the nitride top, ash of amorphous
carbon core, pattern transfer etch to silicon oxide and silicon
oxide trim etch. It would therefore be desirable to employ a
self-aligned spacer approach which avoids the use of deposition
processes for pattern formation and coating of the spacer
layer.
[0008] There is a continuing need in the art for multiple
patterning processes which address one or more of the foregoing
problems associated with the state of the art.
[0009] In accordance with a first aspect of the invention,
self-aligned spacer multiple patterning methods are provided. The
methods comprise: (a) providing a semiconductor substrate
comprising one or more layers to be patterned; (b) applying a first
layer of a first photosensitive composition over the one or more
layers to be patterned, wherein the first photosensitive
composition comprises a first resin component and a first
photoactive component; (c) exposing the first layer to activating
radiation through a patterned photomask; (d) developing the exposed
first layer to form a resist pattern; (e) heat treating the resist
pattern in a hardbake process; (f) treating the hardbaked resist
pattern with a material effective to make alkaline a surface of the
resist pattern; (g) applying a second layer of a second
photosensitive composition over the one or more layers to be
patterned and in contact with the alkaline surface of the resist
pattern, the second photosensitive composition comprising a second
resin component and a photoacid generator; (h) exposing the second
layer to activating radiation; and (i) developing the exposed
second layer to form spacers over the one or more layers to be
patterned, the spacers comprising portions of the second layer not
removed during the second layer development.
[0010] The present invention will be discussed with reference to
the following drawings, in which like reference numerals denote
like features, and in which:
[0011] FIG. 1A-I illustrates in cross-section a first exemplary
process flow for a self-aligned spacer multiple patterning method
in accordance with the invention;
[0012] FIG. 2A-J illustrates in cross-section a second exemplary
process flow for a self-aligned spacer multiple patterning method
in accordance with the invention; and
[0013] FIG. 3A-H illustrates a top-down view of a third exemplary
process flow for a self-aligned spacer multiple patterning method
in accordance with the invention.
[0014] The invention will now be described with reference to FIG.
1A-I, which illustrates in cross-section a first exemplary process
flow for a self-aligned spacer double patterning method in
accordance with the invention.
[0015] FIG. 1A depicts a substrate 100 which may include various
layers and features formed on a surface thereof. The substrate can
be of a material such as a semiconductor, such as silicon or a
compound semiconductor (e.g., III-V or II-VI), glass, quartz,
ceramic, copper and the like. Typically, the substrate is a
semiconductor wafer, such as single crystal silicon or compound
semiconductor wafer, and may have one or more layers and patterned
features formed on a surface thereof. One or more layers to be
patterned 102 may be provided over the substrate 100. Optionally,
the underlying base substrate material itself may be patterned, for
example, when it is desired to form trenches in the substrate
material. In the case of patterning the base substrate material
itself, the pattern shall be considered to be formed in a layer of
the substrate.
[0016] The layers may include, for example, one or more conductive
layers such as layers of aluminum, copper, molybdenum, tantalum,
titanium, tungsten, alloys, nitrides or silicides of such metals,
doped amorphous silicon or doped polysilicon, one or more
dielectric layers such as layers of silicon oxide, silicon nitride,
silicon oxynitride, or metal oxides, semiconductor layers, such as
single-crystal silicon, and combinations thereof. The layers to be
etched can be formed by various techniques, for example: chemical
vapor deposition (CVD) such as plasma-enhanced CVD, low-pressure
CVD or epitaxial growth; physical vapor deposition (PVD) such as
sputtering or evaporation; or electroplating. The particular
thickness of the one or more layers to be etched 102 will vary
depending on the materials and particular devices being formed.
[0017] Depending on the particular layers to be etched, film
thicknesses and photolithographic materials and process to be used,
it may be desired to dispose over the layers 102 a hard mask layer
103 and/or a bottom antireflective coating (BARC) 104 over which a
photoresist layer is to be coated. Use of a hard mask layer may be
desired, for example, with very thin resist layers, where the
layers to be etched require a significant etching depth, and/or
where the particular etchant has poor resist selectivity. Where a
hard mask layer is used, the resist patterns to be formed can be
transferred to the hard mask layer which, in turn, can be used as a
mask for etching the underlying layers 102. Suitable hard mask
materials and formation methods are known in the art. Typical
materials include, for example, tungsten, titanium, titanium
nitride, titanium oxide, zirconium oxide, aluminum oxide, aluminum
oxynitride, hafnium oxide, amorphous carbon, silicon oxynitride and
silicon nitride. The hard mask layer 103 can include a single layer
or a plurality of layers of different materials. The hard mask
layer can be formed, for example, by chemical or physical vapor
deposition techniques.
[0018] A bottom antireflective coating 104 may be desirable where
the substrate and/or underlying layers would otherwise reflect a
significant amount of incident radiation during photoresist
exposure such that the quality of the formed pattern would be
adversely affected. Such coatings can improve depth-of-focus,
exposure latitude, linewidth uniformity and CD control.
Antireflective coatings are typically used where the resist is
exposed to deep ultraviolet light (300 nm or less), for example,
KrF excimer laser light (248 nm), ArF excimer laser light (193 nm),
electron beams and soft x-rays. The antireflective coating 104 can
comprise a single layer or a plurality of different layers.
Suitable antireflective materials and methods of formation are
known in the art. Antireflective materials are commercially
available, for example, those sold under the AR trademark by Rohm
and Haas Electronic Materials LLC (Marlborough, Mass. USA), such as
AR.TM.40A and AR.TM.124 antireflectants.
[0019] A first photosensitive composition is applied on the
substrate over the antireflective layer 104 (if present) to form a
first photosensitive layer 106. The first photosensitive
composition comprises a resin component and a photoactive
component. As used herein, the terms "photosensitive material(s)",
"photosensitive composition(s)" and "photoresist(s)" are used
interchangeably. Suitable photoresist materials are known in the
art and include, for example, those based on acrylate, novolak and
silicon chemistries. Suitable resists are described, for example,
in U.S. Application Publication Nos. US20090117489 A1,
US20080193872 A1, US20060246373 A1, US20090117489 A1, US20090123869
A1 and U.S. Pat. No. 7,332,616. The photoresist materials useful in
the methods of the invention for forming a first resist pattern
include both positive- and negative-acting materials.
[0020] Suitable positive-acting materials include positive-acting
chemically amplified photoresists which undergo a
photoacid-promoted deprotection reaction of acid labile groups of
one or more components of the composition to render exposed regions
of a coating layer of the resist more soluble in an aqueous
developer than unexposed regions. Typical photoacid-labile groups
of the photoresist resins include ester groups that contain a
tertiary non-cyclic alkyl carbon (e.g., t-butyl) or a tertiary
alicyclic carbon (e.g., methyladamantyl) covalently linked to the
carboxyl oxygen of the ester. Acetal photoacid-labile groups also
are typical.
[0021] Suitable negative-acting resists typically will contain a
crosslinking component. The crosslinking component is typically
present as a separate resist component. Amine-based crosslinkers
such as a melamine, for example, the Cymel melamine resins, are
typical. Negative-acting photoresist compositions useful in the
invention comprise a mixture of materials that will cure, crosslink
or harden upon exposure to acid, and a photoactive component of the
invention. Particularly useful negative acting compositions
comprise a resin binder such as a phenolic resin, a crosslinker
component and a photoactive component. Such compositions and the
use thereof are disclosed in European Patent Nos. EP0164248B1 and
EP0232972B1, and in U.S. Pat. No. 5,128,232. Typical phenolic
resins for use as the resin binder component include novolaks and
poly(vinylphenol)s such as those discussed above. Typical
crosslinkers include amine-based materials, including melamine,
glycolurils, benzoguanamine-based materials and urea-based
materials. Melamine-formaldehyde resins are generally most typical.
Such crosslinkers are commercially available, for example: the
melamine resins sold by Cytec Industries under the trade names
Cymel 300, 301 and 303; glycoluril resins sold by Cytec Industries
under the trade names Cymel 1170, 1171, 1172; urea-based resins
sold by Teknor Apex Company under the trade names Beetle 60, 65 and
80; and benzoguanamine resins sold by Cytec Industries under the
trade names Cymel 1123 and 1125. For imaging at sub-200 nm
wavelengths such as 193 nm, typical negative-acting photoresists
are disclosed in International Application Pub. No. WO
03077029.
[0022] The resin of the first photosensitive composition preferably
has functional groups that impart alkaline aqueous developability
to the resist composition. For example, typical are resin binders
that comprise polar functional groups such as hydroxyl or
carboxylate. The resin component is used in the composition in an
amount sufficient to render exposed regions in the case of a
positive-acting material, or unexposed regions in the case of a
negative-acting material, of the composition developable in a
developer solution, such as an aqueous alkaline solution. The resin
component will typically comprise about 70 to about 97 wt % of
total solids of the resist.
[0023] The photosensitive composition further comprises a
photoactive component employed in an amount sufficient to generate
a latent image in a coating layer of the composition upon exposure
to activating radiation. For example, the photoactive component
will suitably be present in an amount of from about 1 to 20 wt % of
total solids of the resist. Typical photoactive components in the
resist compositions are one or more photoacid generator. Suitable
PAGs are known in the art of chemically amplified photoresists and
include, for example: onium salts, for example, triphenyl sulfonium
salts, nitrobenzyl derivatives, sulfonic acid esters, diazomethane
derivatives, glyoxime derivatives, sulfonic acid ester derivatives
of an N-hydroxyimide compound and halogen-containing triazine
compounds.
[0024] A typical optional additive of the resists is an added base,
particularly tetrabutylammonium hydroxide (TBAH), or
tetrabutylammonium lactate, which can enhance resolution of a
developed resist relief image. For resists imaged at 193 nm, a
typical added base is a hindered amine such as diazabicyclo
undecene or diazabicyclononene. The added base is suitably used in
relatively small amounts, for example, about 0.03 to 5 wt %
relative to the total solids.
[0025] Photoresists used in accordance with the invention also may
contain other optional materials. For example, other optional
additives include anti-striation agents, plasticizers and speed
enhancers. Such optional additives typically will be present in
minor concentrations in a photoresist composition except for
fillers and dyes which may be present in relatively large
concentrations, for example, in amounts of from about 0.1 to 10 wt
% based on the total weight of a resist's dry components.
[0026] The photoresists useful in the invention are generally
prepared following known procedures. For example, a resist can be
prepared as a coating composition by dissolving the components of
the photoresist in a suitable solvent, for example, a glycol ether
such as 2-methoxyethyl ether (diglyme), ethylene glycol monomethyl
ether, propylene glycol monomethyl ether; propylene glycol
monomethyl ether acetate; lactates such as ethyl lactate or methyl
lactate; propionates, particularly methyl propionate, ethyl
propionate and ethyl ethoxy propionate; a Cellosolve ester such as
methyl Cellosolve acetate; an aromatic hydrocarbon such toluene or
xylene; or a ketone such as methylethyl ketone, cyclohexanone and
2-heptanone. Typically the solids content of the photoresist varies
between about 2 and 25 wt % based on the total weight of the
photoresist composition. Blends of such solvents also are
suitable.
[0027] The methods of the invention can be used with a variety of
imaging wavelengths, for example, radiation having a wavelength of
sub-400 nm, sub-300 or sub-200 nm exposure wavelength, with I-line
(365 nm), 248 nm and 193 nm being typical exposure wavelengths, as
well as EUV and 157 nm. In an exemplary aspect, the photoresists
are suitable for use with and imaged at a sub-200 nm wavelength
such as 193 nm. At such wavelengths, the use of immersion
lithography is typical although dry processing can be used. In
immersion lithography, a fluid (i.e., an immersion fluid) having a
refractive index of between about 1 and about 2 is maintained
between an exposure tool and the photoresist layer during exposure.
A topcoat layer is typically disposed over the photoresist layer to
prevent direct contact between the immersion fluid and photoresist
layer to avoid leaching of components of the photoresist into the
immersion fluid.
[0028] The photosensitive composition can be applied to the
substrate by spin-coating, dipping, roller-coating or other
conventional coating technique. Of these, spin-coating is typical.
For spin-coating, the solids content of the coating solution can be
adjusted to provide a desired film thickness based upon the
specific coating equipment utilized, the viscosity of the solution,
the speed of the coating tool and the amount of time allowed for
spinning. A typical thickness for the first photosensitive layer
106 is from about 500 to 1500 .ANG.. The first photosensitive layer
can next be softbaked to minimize the solvent content in the layer,
thereby forming a tack-free coating and improving adhesion of the
layer to the substrate. The softbake can be conducted on a hotplate
or in an oven, with a hotplate being typical. The softbake
temperature and time will depend, for example, on the particular
material of the photosensitive layer and thickness. Typical
softbakes are conducted at a temperature of from about 90 to
150.degree. C., and a time of from about 30 to 90 seconds.
[0029] If the first photosensitive layer 106 is to be exposed with
an immersion lithography tool, for example a 193 nm immersion
scanner, a topcoat layer (not shown) can be disposed over the
photosensitive layer 106. Use of such a topcoat layer can act as a
barrier between the immersion fluid and underlying photosensitive
layer. In this way, leaching of components of the photosensitive
composition into the immersion fluid, possibly resulting in
contamination of the optical lens and change in the effective
refractive index and transmission properties of the immersion
fluid, can be minimized or avoided. Suitable topcoat compositions
are commercially available, for example, OPTICOAT.TM. topcoat
materials such as OC.TM. 2000 (Rohm and Haas Electronic Materials)
and are otherwise known in the art, for example, those described in
U.S. Patent Application Pub. No. 2006/0246373A1 and in U.S.
Provisional Application Nos. 61/204,007, filed Dec. 31, 2008. Such
compositions can be applied over the photosensitive layer by any
suitable method such as described above with reference to the
photosensitive compositions, with spin coating being typical. The
topcoat layer thickness is typically .lamda./4n (or an odd multiple
thereof), wherein .lamda. is the wavelength of the exposure
radiation and n is the refractive index of the topcoat layer. If a
topcoat layer is present, the first photosensitive layer 106 can be
softbaked after the topcoat layer composition has been applied
rather than prior to topcoat application. In this way, the solvent
from both layers can be removed in a single thermal treatment
step.
[0030] The first photosensitive layer 106 is next exposed to
activating radiation 108 through a first photomask 110 to create a
difference in solubility between exposed and unexposed regions. For
a positive-acting material, as illustrated, the photomask has
optically transparent and optically opaque regions, the optically
transparent regions corresponding to regions of the photosensitive
layer to be removed in a subsequent development step. For
negative-acting materials, the optically opaque regions would
correspond with portions of the resist layer to be developed away.
The exposure energy is typically from about 1 to 100 mJ/cm.sup.2,
dependent upon the exposure tool and the components of the
photosensitive composition. References herein to exposing a
photosensitive composition to radiation that is activating for the
composition indicates that the radiation is capable of forming a
latent image in the photosensitive composition such as by causing a
reaction of the photoactive component, for example, by producing
photoacid from a photoacid generator compound. The photosensitive
compositions are typically photoactivated by a short exposure
wavelength, particularly a sub-400 nm, sub-300 or sub-200 nm
exposure wavelength, with I-line (365 nm), 248 nm and 193 nm being
typical exposure wavelengths, as well as EUV and 157 nm.
[0031] Following exposure of the first photosensitive layer 106, a
post-exposure bake (PEB) of the photosensitive layer is typically
performed at a temperature above the softening point of the layer.
The PEB can be conducted, for example, on a hotplate or in an oven.
Conditions for the PEB will depend, for example, on the particular
material of the photosensitive layer and thickness. The PEB is
typically conducted at a temperature of from about 80 to
150.degree. C., and a time of from about 30 to 90 seconds.
[0032] The exposed photosensitive layer 106 is next developed to
form a first resist pattern 106' as shown in FIG. 1B. While the
developer material will depend on the particular material of the
photosensitive layer 106, suitable developers and development
techniques are known in the art. Typical developers include, for
example, aqueous base developers such as quaternary ammonium
hydroxide solutions, for example, tetra-alkyl ammonium hydroxide
solutions such as 0.26 N tetramethylammonium hydroxide.
[0033] Following development, the first resist pattern 106' is
heat-treated in a first hardbake process to dry the pattern and
form a hardened resist pattern 106'', as shown in FIG. 1C. This
heat-treatment is believed to facilitate adsorption of the
subsequently applied surface treatment chemistry to the resist
pattern. The hardbake is typically conducted with a hot plate or
oven, and is typically conducted at a temperature of about
150.degree. C. or higher, for example, from about 170 to
180.degree. C., and a time of from about 30 to 120 seconds.
[0034] With reference to FIG. 1D, the hardbaked first resist
pattern 106'' is treated with a material effective to make alkaline
a surface of the resist pattern. The alkaline surface interferes
with reaction during exposure of a subsequently applied
photosensitive layer over the resist pattern. For example, in the
case of a positive-acting photosensitive layer, acid-catalyzed
deprotection reaction is prevented in regions in the immediate
vicinity of the underlying alkaline-treated resist pattern. As a
consequence, portions of the photosensitive layer would remain in
those regions after development.
[0035] While not limited thereto, particularly suitable materials
comprise an alkaline material and a surfactant which is different
from the alkaline material. It is believed that the surfactant
promotes formation of a substantially uniform coating layer of the
second resist over the alkaline material treated resist
pattern.
[0036] The alkaline material can take various forms, and may be in
the form of a solution formed by dissolving a solid compound in a
suitable solvent. Suitable alkaline materials for the resist
pattern treatment include, for example, aqueous base developers
such as quaternary ammonium hydroxide solutions, for example,
tetra-alkyl ammonium hydroxide solutions such as 0.26 Normality (N)
(2.38 wt %) tetramethylammonium hydroxide (TMAH). Solvent materials
used for the alkaline material and otherwise in the compositions
should not dissolve or minimize dissolution of the underlying
photoresist The alkaline material (absent any solvent, e.g., water,
alcohol or the like) is typically present in the compositions in an
amount of from about 1 to 10 wt %, based on the total
composition.
[0037] Suitable surfactants for the resist pattern treatment
compositions include those which exhibit an amphiphilic nature,
meaning that they can be both hydrophilic and hydrophobic at the
same time. Amphiphilic surfactants possess a hydrophilic head group
or groups, which have a strong affinity for water and a long
hydrophobic tail, which is organophilic and repels water. Suitable
surfactants can be ionic (i.e., anionic, cationic) or nonionic.
Further examples of surfactants include silicone surfactants,
poly(alkylene oxide) surfactants, and fluorochemical surfactants.
Suitable non-ionic surfactants for use in the aqueous solution
include, but are not limited to, octyl and nonyl phenol ethoxylates
such as TRITON.RTM. X-114, X-100, X-45, X-15 and branched secondary
alcohol ethoxylates such as TERGITOL.TM. TMN-6 (The Dow Chemical
Company, Midland, Mich. USA). Still further exemplary surfactants
include alcohol (primary and secondary) ethoxylates, amine
ethoxylates, glucosides, glucamine, polyethylene glycols,
poly(ethylene glycol-co-propylene glycol), or other surfactants
disclosed in McCutcheon's Emulsifiers and Detergents, North
American Edition for the Year 2000 published by Manufacturers
Confectioners Publishing Co. of Glen Rock, N.J.
[0038] Nonionic surfactants that are acetylenic diol derivatives
also can be suitable, including such surfactants of the following
formulae:
##STR00001##
wherein R.sub.1 and R.sub.4 are a straight or a branched alkyl
chain having from 3 to 10 carbon atoms; R.sub.2 and R.sub.3 are
either H or an alkyl chain suitably having from 1 to 5 carbon
atoms; and m, n, p, and q are numbers that range from 0 to 20. Such
surfactants are commercially available from Air Products and
Chemicals, Inc. of Allentown, Pa. trade names of SURFYNOL.RTM. and
DYNOL.RTM..
[0039] Additional suitable surfactants for use in coating
compositions of the invention include other polymeric compounds
such as the tri-block EO-PO-EO co-polymers PLURONIC.RTM. 25R2,
L121, L123, L31, L81, L101 and P123 (BASF, Inc.).
[0040] Particularly suitable surfactants include amines, typically
primary and secondary amines, i.e., an amine including one or more
primary amine groups and one or more secondary amine groups,
respectively, and combinations thereof. Tertiary amine groups can
be present in addition to the primary and/or secondary amine
groups. Typically, the amine is a multifunctional amine. The amine
can be a polyamine, such as a diamine, triamine or tetra-amine.
Suitable primary amines include compounds of the following formula
(I):
##STR00002##
wherein R is chosen from optionally substituted alkyl such as
optionally substituted C1 to C6 alkyl, such as methyl, ethyl or
propyl, with ethyl being typical. Other suitable primary amines
include poly(allyl amines) represented by the following formula
(II):
##STR00003##
wherein: R.sub.1 is chosen from hydrogen and optionally substituted
alkyl such as C1 to C3 alkyl; R.sub.2 is chosen from optionally
substituted alkylene such as C1 to C6 alkylene, typically methylene
or ethylene; and n is an integer greater than or equal to 3. In an
exemplary primary amine of the formula (N-II), R.sub.1 is hydrogen
and R.sub.2 is methylene. Other suitable amines include those
represented by the following general formulae (III), (IV) and
(V):
##STR00004##
wherein R.sub.1 and R.sub.2 are each independently a hydrogen atom
or an alkyl group with 1 to 10 carbon atoms, and n is an integer
from 1 to 10. Other suitable amines include the following:
##STR00005## ##STR00006##
Of these, tris(2-aminoethyl)amine (TAEA) is particularly
preferred.
[0041] The surfactant is typically present in the compositions in a
relatively small amount, for example, from 0.01 to 5 wt %, for
example, from 0.01 to 1 wt %, based on the weight of total solids
in the composition (total solids being all compositions components
except solvent carrier).
[0042] The resist pattern treatment compositions can comprise one
or more optional components in addition to the alkaline material
and surfactant components. For example, the compositions can
include one or more solvent in addition to any solvent used for the
alkaline material and surfactant. As described above, solvent
materials used for the alkaline material and otherwise in the
compositions should not dissolve or minimize dissolution of the
underlying photoresist. Suitable solvents will therefore depend on
the particular underlying resist material and may include, for
example, water and alcohols such as n-butanol. The optional
components also include one or more base generator compound, such
as a thermal base generator compound and/or a photobase generator
compound.
[0043] The photoresist pattern treatment compositions can be
prepared by admixture in any order of the alkaline material and
surfactant components, and any additional components such as
solvent and base generator compounds. One or more of the components
can be added as a solid or as a pre-mixed solution using a suitable
solvent.
[0044] Preferably, the alkaline treatment includes treatment with a
quaternary ammonium hydroxide and an amine. The quaternary ammonium
hydroxide material and amine can be simultaneously applied to the
substrate, for example, either from a premixed composition or by
applying the materials simultaneously but separate from one another
in which case the composition is formed in situ. Preferably, the
quaternary ammonium hydroxide material and amine are sequentially
applied in that order. The quaternary ammonium hydroxide and amine
materials can be applied as a liquid, gas or vapor, and can be
applied, for example, by spin-coating, dipping, vapor-coating,
chemical vapor deposition (CVD) or other conventional coating
technique. Of these, spin-coating of liquid materials is typical.
Typically, the quaternary ammonium hydroxide and amine materials
can be applied as aqueous solution(s). Where the quaternary
ammonium hydroxide and amine are applied simultaneously, the
surface treated substrate can be rinsed, for example, with
deionized water. Where the quaternary ammonium hydroxide and amine
materials are sequentially applied, the amine can be applied as an
aqueous solution, functioning also as a water rinse. The surface
treated substrate can optionally be rinsed, for example, with
deionized water to remove excess composition.
[0045] A critical dimension (CD) of the first resist pattern 106''
becomes slightly reduced as a result of the surface treatment as
compared with the original CD of resist pattern 106'. This CD loss
is believed to be attributed to further development of the first
resist pattern during the surface treatment. The surface treatment
forms a modified first resist pattern surface 112 which is alkaline
and which has a line width roughness less than that of the
pre-treated surface.
[0046] A second photosensitive composition as described above is
coated over the first resist pattern 106'' and BARC layer 104 to
form a second photosensitive layer 114, as shown in FIG. 1E. The
alkaline surface on the resist pattern is believed to interfere
with photoreaction during exposure in a subsequently applied
photoresist layer over the resist pattern. For example, in the case
of a positive-acting second photosensitive layer, a deprotection
reaction is prevented in that layer in regions in the immediate
vicinity of the underlying alkaline-treated resist pattern. As a
consequence, a certain amount of the second photosensitive layer
would remain in those regions after development.
[0047] Except as otherwise stated, the second photosensitive
composition can be applied and processed in the same manner
including the materials and conditions described above with respect
to the first photosensitive layer. While the first photosensitive
layer 106 can be positive- or negative-acting, the second
photosensitive layer 114 is typically positive-acting. In the
illustrated process flow, both the first and second photosensitive
compositions are positive acting. In such a case, different
materials for the first and second photosensitive layers can be
employed such that the first resist pattern 106'' is not removed
during processing of the second photosensitive layer 114. For
example, a first photosensitive layer may be chosen which has a
characteristic post-exposure bake temperature that is higher than
that of the second photosensitive layer. In this way, the exposed
regions of the second photosensitive layer can be selectively
removed without also removing exposed regions of the first resist.
It may, however, be desired to remove the first resist pattern
106'' at the same time the second photosensitive layer 114 is
developed. In this case, the same or similar material can be used
for the first and second photosensitive layers, or the second
photosensitive composition can be selected to have a higher
characteristic post-exposure bake temperature than that of the
first photosensitive composition. The process flow in the case of
simultaneous development of the first resist pattern 106'' and
second photosensitive layer 114 would skip from that shown in FIG.
1E to 1G.
[0048] The second photosensitive layer 114 is typically coated to a
thickness less than that of the surface-treated first resist
pattern 106'' to facilitate removal of the first resist pattern
106'' in a subsequent step as described below. Also for purposes of
removing the first resist pattern 106'', the etchant to be used
should have good selectivity to the first photosensitive pattern
relative to the second photosensitive composition. As such, the
first and second photosensitive compositions should be different.
It may, for example, be desired to use a silicon-based polymer,
such as a silsesquioxane-type polymer, for the first or second
photosensitive composition, while using a photosensitive
composition of a different chemistry, such as a composition which
is free of silicon resins, for example, an acrylate-, novolak- or
phenolic-type photoresist, for the other of the first or second
photosensitive composition.
[0049] The second photosensitive layer 114 can next be softbaked.
If the second photosensitive layer 114 is to be exposed with an
immersion lithography tool, a topcoat layer (not shown) as
described above can be disposed over the second photosensitive
layer 114. If a topcoat layer is used, the second photosensitive
layer 114 can be softbaked after the topcoat layer composition has
been applied rather than prior to its application.
[0050] With reference to FIG. 1(E), the second photosensitive layer
114 is exposed to activating radiation 108, typically by
flood-exposure, i.e., without use of a patterned photomask. The
exposed second photosensitive layer is heat-treated in a
post-exposure bake and developed. The alkaline-modified surface
region 112 of the first resist pattern 106'' prevents photoreaction
in the second resist layer 114 in the vicinity of the surface
region. As a result, unreacted portions of the second
photosensitive composition remain as spacers 114' on sidewalls of
the first resist pattern 106'', as illustrated in FIG. 1F. At this
point, if desired to adjust the width of the spacers, for example
to increase the width, a series of steps in the process can be
repeated one or more times beginning with the first hardbake
through development of an additional photosensitive layer of the
second photosensitive composition, as indicated by the dashed arrow
in FIG. 1.
[0051] The first photoresist pattern 106'' is next removed, leaving
behind spacers 114', as shown in FIG. 1G. Because the spacers are
typically formed on all side surfaces of the first resist pattern
with the resist pattern at the center, they generally result in a
closed-ring structure. Therefore, in the case of fabricating a line
pattern using the spacer, a trimming process may be performed to
remove ends of the patterns to separate the spacer into a discrete
line pattern. The trimming process can be conducted, for example,
using known etching techniques.
[0052] The BARC layer 104 is selectively etched using the spacers
114' as an etch mask, exposing the underlying hardmask layer 103.
The hardmask layer is next selectively etched, again using the
spacers 114' as an etch mask, resulting in patterned BARC and
hardmask layers 104', 103', as shown in FIG. 1H. Suitable etching
techniques and chemistries for etching the BARC layer and hardmask
layer are known in the art and will depend, for example, on the
particular materials of these layers. Dry-etching processes such as
reactive ion etching are typical. The spacers 114' and patterned
BARC layer 104' are next removed from the substrate using known
techniques, for example, oxygen plasma ashing.
[0053] Using the hardmask pattern 103' as an etch mask, the one or
more layers 102 are selectively etched. Suitable etching techniques
and chemistries for etching the underlying layers 102 are known in
the art, with dry-etching processes such as reactive ion etching
being typical. The patterned hardmask layer 103' can next be
removed from the substrate surface using known techniques, for
example, a dry-etching process such as reactive ion etching. The
resulting double patterned structure is a pattern of etched
features 102' as illustrated in FIG. 1I. In an alternative
exemplary method, it may be desirable to pattern the layer 102
directly using the spacers 114' without the use of a hardmask layer
103. Whether direct patterning with the spacers can be employed
will depend on factors such as the materials involved, resist
selectivity, resist pattern thickness and pattern dimensions.
[0054] FIG. 2 illustrates in cross-section a further exemplary
process flow for a self-aligned spacer multiple patterning method
in accordance with the invention. This process flow is a variation
of the double patterning technique described with reference to FIG.
1, in which one or more additional spacer 114a' is formed on each
original spacer 114' to allow a further decreased line pitch.
Except as otherwise stated, the description above with respect to
FIG. 1 is also applicable to the process flow of FIG. 2.
[0055] The process with respect to FIG. 2A-F is carried out in the
manner described above with reference to FIG. 1A-F. After formation
of spacers 114' as shown in FIG. 2F, the sequence of steps
beginning with the first hardbake through development is repeated
except that the next (third) photosensitive layer is of a
composition having the same or similar etch selectivity as the
first photosensitive layer 106, as the first resist pattern and
pattern 106''a formed from the third photosensitive layer are to be
removed at the same time. The third photosensitive layer should
additionally be positive-acting. After development of the exposed
third photosensitive layer, second spacers 106a' remain on exposed
sides of the originally formed spacers 114'. The same sequence of
steps is again repeated, this time forming a fourth positive-acting
photosensitive layer of a composition having the same or similar
etch selectivity as the second photosensitive layer 114. The
resulting structure after development, shown in FIG. 2G, includes
third spacers 114a' formed on exposed sides of second spacers
106a'. This spacer formation process can optionally be repeated one
or more additional times, alternating between photosensitive layer
chemistries of the first and second types to create additional
spacers of the two types in a sandwich structure. The photoresist
patterns 106'', 106a'' from the first photosensitive composition
type are next removed, leaving behind the spacers 114', 114a', as
shown in FIG. 2H. In the same manner described above with reference
to FIG. 1, the spacers 114', 114a' are used as an etching mask to
pattern one or more underlying layers directly or to first pattern
an underlying hardmask layer which is then used to pattern the
underlying layers, as illustrated in FIG. 2I-J.
[0056] FIG. 3 is a top-down view of a further exemplary process
flow for a self-aligned spacer multiple patterning method in
accordance with the invention. This process flow is a further
variation of the technique described with reference to FIG. 1,
illustrating a triple patterning technique using a hybrid approach
in which both photolithographic and film deposition techniques are
used to form spacers. Except as otherwise stated, the description
above with respect to FIG. 1 is also applicable to the process flow
of FIG. 3.
[0057] FIG. 3A illustrates the substrate after formation of the
first resist pattern 106'. The structure following alkaline surface
treatment which results in modified first resist pattern surface
112 is shown in FIG. 3B, and after coating of the second
photosensitive layer 114 is shown in FIG. 3C. FIG. 3D illustrates
the substrate after softbake of the second photosensitive layer
114. The softbake causes the alkaline material to diffuse into a
portion of both the first resist pattern and second photosensitive
layer. The extent of diffusion can be controlled based on the
softbake temperature and the diffusion coefficient of the alkaline
material in the first resist pattern and second photosensitive
layer. For example, if the same or similar material is used for
both the first and second photosensitive layers, the extent of
diffusion will be about the same in the first resist pattern and
second photosensitive layer. The result of this diffusion is an
enlarged alkaline region 112' in the first resist pattern and
second photosensitive layer. The first resist pattern 106' and
second photosensitive layer 114 are exposed and developed, leaving
spacers 114' as shown in FIG. 3E. The spacers correspond to those
portions of the first resist pattern 106' and second photosensitive
layer 114 within the alkaline region 112'. In this exemplified
process, those portions of the first resist pattern and second
photosensitive layer not "poisoned" by the alkaline material are
removed at the same time in the development step. This can be
accomplished by using a post-exposure bake temperature after the
exposure which is the same or greater than the characteristic
post-exposure bake temperature of the first and second
photosensitive layers. As described below, spacers 114' are used as
a pattern or template on which to form additional spacers that are
used for patterning of one or more underlying layers.
[0058] The spacers 114' can optionally be trimmed to reduce their
width, resulting in trimmed spacers 114'' as illustrated in FIG.
3F. Suitable trimming processes are known in the art. A spacer
layer is deposited over the spacers 114'', for example, by chemical
vapor deposition such as plasma-enhanced CVD, low-pressure CVD or
epitaxial growth, atomic layer deposition or sputtering. Of these,
chemical vapor deposition is typical. Suitable materials for the
spacer layer include, for example, silicon oxide, silicon nitride
or silicon oxynitride. The thickness of the spacer layer will
depend, for example, on the desired dimensions of the structures
being formed and the degree of conformality of the spacer layer. A
typical thickness for the spacer layer is from about 100 to 10001.
Because the spacer layer is typically coated over the entire
substrate surface, etching is conducted to remove the spacer layer
material on horizontal surfaces of the substrate, leaving material
as spacers 116 on sidewalls of the trimmed spacer patterns 114'' as
depicted in FIG. 3G. The sacrificial spacers 114'' are removed,
leaving behind only the sidewall spacers 116 to be used as a mask
for etching one or more underlying layers. The trimmed sacrificial
spacer patterns 114'' are removed, for example, by oxygen plasma
ASH. The resulting structure is trimmed, for example, along the
dashed lines shown in FIG. 3G to separate the spacers 116 into
discrete lines as shown in FIG. 3H. Etching of the one or more
underlying layers can be conducted as described above with
reference to the process flow of FIG. 1. Because there are two
spacers for every line (i.e., one on each side surface) and two
spacer processes are employed, the line density can effectively be
quadrupled by double pitch-splitting.
EXAMPLES
Example 1
L1 Resist Polymer (Poly(IAM/.alpha.-GBLMA/ODOTMA/HAMA))
Synthesis
[0059] 10.51 grams (g) of 2-methyl-acrylic acid
1-isopropyl-adamantanyl ester (IAM), 6.82 g of 2-methyl-acrylic
acid 2-oxo-tetrahydro-furan-3yl ester (.alpha.-GBLMA), 6.36 g of
2-methyl-acrylic acid
3-oxo-4,10-dioxa-tricyclo[5.2.1.0.sup.2,6]dec-8-yl ester (ODOTMA)
and 6.31 g of 2-methyl-acrylic acid 3-hydroxy-adamantanyl ester
(HAMA) were dissolved in 27 g of tetrahydrofuran (THF). The mixture
was degassed by bubbling with nitrogen for 20 minutes. A 500 ml
flask equipped with a condenser, nitrogen inlet and mechanical
stirrer was charged with 11 g of THF, and the solution was brought
to a temperature of 67.degree. C. 5.23 g of
dimethyl-2,2-azodiisobutyrate (17 mol % based on total monomers)
was dissolved in 5 g of THF and charged into the flask. The monomer
solution was fed into a reactor at a rate of 16.0 milliliters per
hour (mL/h) for 3 hours 30 minutes. The polymerization mixture was
then stirred for an additional 30 minutes at 67.degree. C. 5 g of
THF was next added to the reactor and the polymerization mixture
was cooled to room temperature. Precipitation was carried out in
1.0 L of isopropyl alcohol. After filtration, the polymer was
dried, re-dissolved in 50 g of THF, re-precipitated into 1.1 L of
isopropyl alcohol, filtered and dried in a vacuum oven at
45.degree. C. for 48 hours, resulting in 25.4 g of the
poly(IAM/.alpha.-GBLMA/ODOTMA/HAMA) polymer (Mw=7,934 and
Mw/Mn=.about.1.46) shown below:
##STR00007##
L1 Resist Formulation
[0060] 3.169 g of the polymer formed as described above was
dissolved in 96.38 g of a solvent mixture of 70 wt % propylene
glycol monomethyl ether acetate (PGMEA) and 30 wt % cyclohexanone.
To this mixture was added 0.405 g of triphenylsulfonium
(adamantan-1yl methoxycarbonyl)-difluoro-methanesulfonate, 0.041 g
of 1-(tert-butoxycarbonyl)-4-hydroxypiperidine and 0.005 g of
POLYFOX.RTM. PF-656 surfactant (Omnova Solutions Inc.). The
resulting mixture was rolled on a roller for six hours and then
filtered through a Teflon filter having a 0.2 micron pore size,
thereby forming a positive-acting photoresist composition.
Surface Treatment Solution Formulation
[0061] A surface treatment solution was prepared by mixing 5 g of 1
wt % tris(2-aminoethyl)amine (TAEA) (Sigma-Aldrich) solution in
deionized water, 1 g of 10 wt % of a surfactant solution (TERGITOL
TMN-6, The Dow Chemical Company, Midland, Mich., USA), and 194 g of
deionized water. This solution was filtered through a nylon filter
having a 0.1 micron pore size.
First Lithography (L1) Patterning of Lines and Spaces
[0062] A 300 mm silicon wafer was spin-coated with AR.TM.40A
antireflectant (Rohm and Haas Electronic Materials LLC) to form a
first bottom antireflective coating (BARC) on a TEL CLEAN TRACK.TM.
LITHIUS.TM. i+ coater/developer. The wafer was baked for 60 seconds
at 215.degree. C., yielding a first BARC film thickness of 75 nm. A
second BARC layer was next coated over the first BARC using
AR.TM.124 antireflectant (Rohm and Haas Electronic Materials), and
was baked at 205.degree. C. for 60 seconds to generate a 23 nm top
BARC layer.
[0063] The L1 photoresist composition formed as described above was
coated on top of the dual BARCs and soft-baked at 110.degree. C.
for 60 seconds, resulting in a resist film thickness of 750 .ANG..
The first resist layer was coated with a topcoat layer (OC.TM.2000
topcoat material, Rohm and Haas Electronic Materials) and exposed
at various doses from 15 to 75 mJ/cm.sup.2 through a reticle having
various critical dimensions using an ASML TWINSCANT.TM. XT:1900i
immersion scanner with a numerical aperture of 1.35 and dipole-35Y
illumination (0.96 outer sigma/0.76 inner sigma) with
X-polarization. The wafer was then post-exposure baked (PEB) at
100.degree. C. for 60 seconds and developed for 12 seconds using
Microposit.TM. MF CD-26 developer (Rohm and Haas Electronic
Materials) to render first lithography (L1) patterns.
Curing and Surface Treatment
[0064] The wafer was hardbaked at 180.degree. C. for 60 seconds.
The wafer was next exposed to surface treatment chemistry in a
sequential process by which the wafer was first rinsed with 2.38 wt
% TMAH in water solution for 12 seconds using a TEL GP nozzle, and
then rinsed with the surface treatment solution formulation
described above on the TEL wafer track with wafer spin.
Second Lithography (L2)
[0065] EPIC.TM. 2098 positive photoresist (Rohm and Haas Electronic
Materials) was coated over the surface-treated L1 patterns on the
coater/developer at a spin speed that would provide a film
thickness of 450 .ANG. on a bare silicon wafer. The wafer was
soft-baked at 120.degree. C. for 60 seconds, and then coated with
OC.TM. 2000 topcoat material (Rohm and Haas Electronic Materials).
The second lithography (L2) was carried out using the same scanner
settings as in the first lithographic process but using flood
exposure with no mask at various exposure doses of from 12 to 34
mJ/cm.sup.2. The wafer was then post-exposure baked (PEB) at
110.degree. C. for 60 seconds and developed for 12 seconds using
Microposit.TM. MF CD-26 developer (Rohm and Haas Electronic
Materials). Spacer structures were thereby formed.
Examples 2-18
[0066] The procedures described above with respect to Example 1 are
repeated except using the components set forth below in Table 1 in
place of the TAEA and TERGITOL TMN-6 surfactant.
TABLE-US-00001 TABLE 1 Example Amine Surfactant 1 A-1 TERGITOL
TMN-6 2 A-2 TERGITOL TMN-6 3 A-3 TERGITOL TMN-6 4 A-4 TERGITOL
TMN-6 5 A-5 TERGITOL TMN-6 6 A-6 TERGITOL TMN-6 7 A-7 TERGITOL
TMN-6 8 A-8 TERGITOL TMN-6 9 A-9 TERGITOL TMN-6 10 A-1 -- 11 A-2 --
12 A-3 -- 13 A-4 -- 14 A-5 -- 15 A-6 -- 16 A-7 -- 17 A-8 -- 18 A-9
--
It is expected that spacer structures would result after L2
development.
* * * * *